The present invention relates to chirp switching circuits and optical transmission systems and more particularly to a chirp switching circuit for switching transmission chirp and an optical transmission system that uses such a chirp switching circuit.
In view of the needs of increasing transmission capacity, technical development is being undertaken with regard to optical transmission systems in the directions of achieving: higher transmission speed (increase of bit rate); wavelength multiplexing for enabling transmission of plural signals via a single optical fiber at different wavelengths; and increase of transmission distance.
With increase in the transmission speed, there occurs a distortion of signal waveform in the optical signals transmitted through an optical transmission path because of spreading of the spectrum of the optical signals resulting from signal modulation and further from the dispersion pertinent to the signal transmission path. Such distortion of the signal waveform limits the distance that the signal can be transmitted. This distortion of transmitted signal waveform depends upon the mutual relationship between the chirp (time-dependent wavelength shift) that is pertinent to the transmission part and the chirp caused by the dispersion of the transmission path and non-linearity of the transmission path.
In a Mach-Zehnder (MZ) modulator, it is possible to adjust the chirp (time-dependent wavelength shift) of the transmitter output by changing the ratio of phase modulation between the waveguides of two directions. The chirp caused by the transmitter induces a pulse compression or decompression in cooperation with dispersion and chirp of the transmission path, and thus, it is possible to relax the foregoing constraint imposed upon the transmission path length because of dispersion, by suitably choosing a transmission output chirp according to the transmission path condition.
Meanwhile, there arises a situation in which it is desired to switch the chirp of the Mach-Zehnder modulator, as in the case of switching the signal path from the current path to a path where the transmission path conduction such as transmission distance is different. FIGS. 1A and 1B show such an example of switching the signal path from a current (work) path to a reserve (protection) path in an optical transmission system of ring structure.
Referring to FIGS. 1A and 1B, the optical transmission system is constructed by nodes 10a-10f connected in the form of a ring, wherein each of the nodes 10a-10f achieves optical amplification and compensation of dispersion. Thus, when an optical signal is to be transmitted from a transmission part 11 to a receiving part 12, the node 10b usually chooses a current path A that reaches the node 10b from the node 10a in a clockwise direction as shown in FIG. 1A.
On the other hand, when there occurs a failure in the optical transmission path between the nodes 10a and 10b, the transmission path is switched to a reserve path B that reaches from the node 10a to the node 10d in the anti-clockwise direction via the nodes 10f, 10e, 10d and 10c. 
Here, let us assume that the path A has a negative residual dispersion and the path B has a positive residual dispersion.
As can be seen from FIG. 3 that shows the relationship between the residual dispersion and the sign of the chirp, use of positive chirp is suited in the case there is a negative residual chirp, while in the case the residual dispersion is positive, it is suitable to use a negative chirp. Thus, when there is a need to switch the transmission path from the path A to the path B, there is a need of switching the transmission chirp from positive to negative.
FIG. 4 shows an example of the chirp switching circuit that uses a conventional Mach-Zehnder optical modulator.
Referring to FIG. 4, the Mach-Zehnder optical modulator divides the light coming in from an optical source 15 into two, different optical waveguides 17 and 18 at a Y-branching part 16, wherein the respective lights undergo phase change in the optical waveguides 17 and 18 and are merged again at a Y-branched part 19. Thereby, On/Off control is achieved for the optical signal based on the phase difference thus induced.
FIG. 5A shows input/output characteristics of the chirp switching circuit that uses such a Mach-Zehnder optical modulator, while FIGS. 5B-5F show input/output signal waveforms of the chirp switching circuit.
In the Mach-Zehnder modulator, the logic value of an input signal (electric signal) shown in FIG. 5B is inverted by a high-speed driver circuit 21 and the operational point of the Mach-Zehnder modulator is shifted by an operational point stabilization circuit 22 at the same time. With this, there is caused inversion in the transmission chirp. It should be noted that FIG. 5B shows the optical intensity in the optical waveguide 17 while FIG. 5C shows the optical intensity in the optical waveguide 18. Further, FIG. 5D shows the phase of the output optical signal while FIG. 5F shows the chirp caused in the output optical signal.
Because the optical amplifier in each of the nodes 10a-10f has a time-dependence in the response characteristics thereof, there occurs a change of output when there is caused a change in the input, wherein there are cases in which such a change is accumulated. It should be noted that such accumulated change of optical signals may cause overload or even damage in the optical components at the reception side or at the intermediate position on the signal transmission path. Thus, in the case there is provided optical amplifiers in the transmission path and in the case there is going to be made switching of chirp in correspondence to the switching of transmission path from the current transmission path to a transmission path of different path condition, it is desired that the change of optical output caused with the switching of chirp is suppressed as much as possible.
Parent Reference 1 teaches the technology of obtaining a chirped optical signal of inverted polarity by changing the operational point of the Mach-Zehnder optical modulator in correspondence to the chirp polarity.
PATENT REFERENCE 1 Japanese Laid-Open Patent Application 11-266200